Sunday, July 26, 2009

Little-leaf sensitive brier: Native relative of nonnative mimosa trees a rare delight in Northwest Arkansas lowlands such as WPWP

Please click on image of sensitive brier, also known as Mimosa microphylla and Schrankia microphylla
Mimosa microphylla, the Little-leaf Sensitive Brier--a plant whose flowers are delightful to behold but whose real claim to fame is its behavior. "Plant BEHAVIOR?" you ask. Yes, plant behavior.
M. microphylla grows naturally in the southeastern U.S. as far west as Texas; it is considered "very rare" and subject to possible extirpation in Virginia. And while Asian Mimosa is a woody tree that reaches 40 feet in height, its smaller local namesake is a perennial herbaceous vine whose green stems wind along the ground or ramble over low walls and shrubs. The 3-to-6-foot stems grow from a taproot that allows Little-leaf Sensitive Brier to overwinter underground and get a head start when warm days of spring arrive. We found our first plant at Hilton Pond Center back in the mid-1980s in a spot that is now too-shaded by new-growth trees; the specimen depicted on this page is in full sun on the Center's road frontage--right where road crews are likely to whack it back next time they cut vegetation along the shoulder. One vine from the taproot grows flat along the substrate, while the other has managed to find vertical support from low branches of an Eastern Red Cedar, Juniperus virginiana (large bottom photo).

The pink "powderpuff" blossom of Little-leaf Sensitive Brier is reminiscent of that of Mimosa trees, but to our eye it has much better form. Perfectly spherical and about three-quarters of an inch across, the flower heads erupt along the stem and are quite complex. Each globose cluster consists of many individual flowers: Sets of five petals fused into tubes; long anther stalks; and multiple stamens that carry a tiny spot of pollen at their tips. The overall effect is such that the entire blossom resembles a burst of fireworks, or--as one observer put it--the flower of Little-leaf Sensitive Brier looks like it could have been designed by Disney studios.

Both Mimosa and Little-leaf Sensitive Brier are classified in the Fabaceae (Pea Family), so one should not be surprised to find that seeds from each are borne in pods. Mimosa's fruit ripens in a more typical pea pod, while that of Little-leaf Sensitive Brier is produced in a 3-6" spiny, downy, awl-shaped structure in which lies a row of tiny seeds. Members of the Pea Family are also known as "legumes"--plants that harbor nitrogen-collecting bacteria in nodes on their roots. Like soybeans and other legumes, Mimosa and Little-leafed Sensitive Brier help enrich land on which they grow by returning nitrogen to the soil.

Incidentally, one problem native plant enthusiasts will encounter when seeking information about Little-leaf Sensitive Brier is that it has been known by a confusing variety of scientific names. First described by Swedish botanist Jonas C. Dryander in the late 18th century, it was "misidentified" or "rediscovered" so many times that the scientific literature lists a dozen synonyms--the most common being Schrankia microphylla.

To learn more about the subject of this week's photo essay, we also need to dissect its common name: Little-leaf Sensitive Brier. Like many legumes, this plant has compound leaves, but in this case they are doubly compound; i.e., the petiole comes off the stems and is divided into leaflets that, in turn, are divided into subleaflets. These subleaflets are less than a quarter-inch long--hence the name "Little-leaf." The "Brier" part of the name pertains to inconspicuous decurved thorns (above right) that grow along the main vine, on the flower stalk, and on the midrib of the main leaf petiole. In spite of being less than an eighth-of-an-inch long, these briers are quite sharp and can easily lacerate a bare ankle or ungloved hand.

No comments: